1. Technical Field
The present invention relates to methods and apparatus for determining a microorganism's sensitivity to a growth altering substances in general, and to methods and apparatus for determining the minimum concentration at which a growth altering agent has an appreciable effect on a microorganism in particular.
2. Background Information
Growth-altering agents like antibiotics, antiseptics, drugs, hormones, mutagens, and nutrients can be used to alter, inhibit, or enhance microbial growth. Because the effect of a growth-altering agent is typically a function of its concentration, there is considerable utility in knowing the concentration at which a growth-altering agent will have an appreciable effect on a microorganism. Some existing methods and apparatus for evaluating the effect a growth-altering agent has on a microorganism expose the microorganism to a plurality of discrete concentrations of a growth-altering agent. The Kirby-Bauer test, for example, utilizes a number of disks placed on a layer of growth medium, each of which contains antibiotic in a specific concentration. Bacteria grown on the growth medium form a visible coating, except in the area around those disks having sufficient antibiotic concentration to inhibit bacterial growth. A disadvantage of the Kirby-Bauer test is that there are a number of variables which affect the antibiotic concentration at any given point in the growth medium, and thus do not readily allow for an accurate determination of the minimum inhibitory concentration of the antibiotic.
Another method and apparatus for evaluating the effect a growth-altering agent has on a microorganism is the tube dilution method wherein an equal amount of target microorganism is placed in a plurality of wells (referred to as "tubes") disposed in a platter, and different concentrations of a growth-altering agent are added to each tube. At some concentration of growth-altering agent, there will be an appreciable change in the target microorganism; e.g., its growth will be altered, enhanced, or inhibited. A disadvantage of the tube dilution method is that its accuracy depends on the step size in concentration change between tubes. A small step size yields greater accuracy, but may require an impractical number of tubes and effort. In addition, preparing accurate dilutions is an expensive process that increases in cost with the number of tubes. Hence, increasing the accuracy of this method can also increase the cost and time required.
What are needed are a method and an apparatus for determining the concentration at which a growth-altering agent has an appreciable effect on a target microorganism, and one that can do so accurately in a time effective manner.